Recent years have witnessed a growing importance of sealed type secondary cells such as lithium-ion cells, for instance as vehicle on-board power sources, and as power sources in personal computers, cell phones and the like. Lithium-ion cells, in particular, are lightweight and afford high energy densities, and are thus expected to become preferred high-output on-board power sources.
A typical method for manufacturing a sealed type cell of the above kind involves housing, in a metallic external case, a power-generating element (electrode body) that is provided with a positive electrode, having a positive electrode active material, and a negative electrode, having a negative electrode active material, together with an appropriate electrolyte, followed by (hermetic) sealing of the case. Herein, the cell (cell assembly) immediately after assembly is yet uncharged, and hence initial charging (first-time charging of the built cell, likewise hereafter) is performed on the cell assembly. Some gas (for instance, hydrogen gas) is ordinarily generated in the cell during initial charging, whereupon the internal pressure of the cell rises as a result. Therefore, the gas in the case is let out, after initial charging, by temporarily removing the hermetically sealed state. Such gas relieving (gas discharge) is typically accomplished by way of a through-hole provided in the case, and by forming a gas discharge passage that communicates the interior and the exterior of the case. Once discharge of the gas is over, the gas discharge passage is sealed, whereby the interior of the case is reverted to a hermetically sealed state. Sealed type cells are thus manufactured as described above. Examples of background art documents relating to this technology include, for instance, Patent documents 1 to 4 below.
Patent document 1: Japanese Patent Application Publication No. 2004-355977
Patent document 2: Japanese Patent Application Publication No. 2001-338674
Patent document 3: Japanese Patent Application Publication No. 2007-193969
Patent document 4: Japanese Patent Application Publication No. 2005-339946